Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.
Drilling to detect gold-bearing alluvium beneath the basalt cover has, for over 100 years, been the only means of prospecting for deep leads. These holes, usually diamond drilled, provide little data other than the thickness of basalt cover and alluvium, and the depth of bedrock. The presence of gold was often not able to be determined.
Determining the location and boundaries of deep lead gold deposits and the assessment of their economic potential is difficult, and the results are considerably less definitive than for many other types of mineral deposits.
Generally speaking, for a deep lead, the mineralised target zone is narrow relative to the breadth of the original valley floor, and more so compared with the width of the basalt sheet which later flooded the valley, sometimes obscuring it entirely. It is impossible therefore, to use surface geological evidence to trace the course of the narrow and sinuous river bed which carries the auriferous gravel deposits. The only practical means of exploration has been the drilling of lines of holes across the presumed likely course of the lead. This is effective in outlining the general profile of the original valley floor, and in locating thicker accumulations of wash. However, the cost of a drill pattern of sufficiently close spacing to detect bedrock gutters of only a few tens of meters width has been regarded as prohibitive.
The general experience of using drill-hole results to determine gold concentrations in the wash has proved quite unreliable. To penetrate the basalt cover, small diameter drilling (200 mm and less) has had to be used. Holes of this size provide a sample volume much too small for reliable grade determination in poorly consolidated gravel and sand which contains coarse gold particles. The larger gold grains are often not recovered by the drilling method and frequently, results appear to have understated even the local grades within a deposit. Modern exploration geophysical methods have been applied with mixed success in determining depths of basalt and bedrock in deep lead areas.
The distribution of gold particle size along the trend of the lead is of significance in assessing the likelihood that the lead may have been charged at several points along its course with gold derived from tributary streams, or from bedrock reef outcrops which it may have traversed. Geologically, the introduction of gold at various points along its course is expected to be considerably more favourable for the downstream persistence of economic concentrations, than if the only source was at the head of the lead.
Any mining method which proposes to exploit these deep lead deposits will have to operate through a depth of cover of up to 120 m. Most commonly this consists of fairly competent and free-standing basalt. The immediate hanging wall to the gold bearing wash is often a poorly consolidated and heavily water-bearing section, in the range of up to 100 m thick, of sand and clay which carries little, if any gold. The specially developed underground mining methods of the early miners were able to extract the thin (0.5 to 1.5 m) layer of gold bearing wash without suffering significant dilution from these overlying sand and clay beds. The development of a method which can profitably mine these deeps leads without the requirement for intensive underground labour would constitute a major technological advance.
A number of methods have been proposed for mining from boreholes. The basis of these has generally been the use of water-jets to break up the alluvium and the pumping the resulting slurry to the surface. The problem with these systems is basically one of cost. In an unconsolidated deposit such as a Deep Lead the lower grade roof will feed into the cavern. This means that a large volume of “overburden” will be extracted to extract a small volume of wash. The water jet is likely to have a short range as it is operating in water which will disperse the force. The net result is that a large number of boreholes are required to extract the wash and the economics are unlikely to be favourable. Furthermore, there are environmental pressures which limit the types of methods that can be employed to extract the gold—regardless of how safe a process could be proven to be in many cases they are unacceptable to the local community.
In light of the above, it would be advantageous to have mechanisms for extracting desirable minerals from underground alluvial deposits that are contained beneath a rock layer, such as a basalt layer.